A couple of top secret files have been leaked thanks to spokesman Oishii Jirou of Kojima Productions. The images show the original logos for Peace Walker and, huge surprise here, the logos reveal that the game is indeed the next canon title in the Metal Gear timeline.
Peace Walker was originally planned to be announced as Metal Gear Solid 5: Peace Walker, however it seems that the number was dropped prior to the announcement at the Electronic Entertainment Expo. Even so, Kojima himself has reiterated on multiple occasions that Peace Walker is the true sequel in the Metal Gear saga and it'll be just as big, if not bigger, than Guns of the Patriots.
Big Boss won't see his next outing until April in the land of the rising sun and May for the rest of the world.
Source: The KP Blogcast